1. Field of the Invention
This invention relates to a broadband switching system for the switching of asynchronously transferred data cells, and to a method of switching asynchronously transmitted data cells.
2. Related Art
Broadband switching networks for switching asynchronously transferred cells are known, in which a predetermined level of bandwidth is allocated to a transmission channel connecting a first customer to a second customer. In some of these known systems, a communications channel is provided over a significant period of time, effectively of the leased-line type, and manual measures are implemented in order to set up such a connection or to modify a connection according to the particular terminations and the level of traffic being conveyed. Consequently, it is usual for customers to incur a fixed rate charge as part of the overall charge for the connection, resulting in payment being made irrespective as to whether the connection is being used or not.
Alternative systems have been proposed or are available. In particular, it is possible for connections to be established on a dial-up basis, requiring termination equipment to be provided with facilities for establishing connections by issuing signalling commands and responding to similar commands issued by the network.
The use of permanent circuits to support a private communications network is widespread. The demand for such circuits is expected to grow to include broadband rates above 2 Mbit/s, the circuits carrying traffic multiplexed from sources which are inherently bursty, possibly together with traffic which is transmitted at constant bit rates and is delay sensitive, such as voice transmission and constant bit-rate video.
Asynchronous transfer mode (ATM) cells all have a fixed information field of forty eight octets which can carry customer traffic or customer-originating control information (signalling). These two types of data transmission are distinguished by setting virtual path (VP) and vertical circuit (VC) values in the cell headers. Another field provided in the ATM header is known as cell loss priority, which enables low priority cells to be distinguished from high priority cells. In the event of congestion, the low priority cells may be discarded first.
For private circuits within an ATM based network, the desired route, the required bandwidth, and the quality of service (QOS) are set up using network management procedures. The private circuits are known as permanent virtual circuits (PVCs) because there is no actual physical circuit, only a VP/VC value or xe2x80x9clabelxe2x80x9d which is associated with information stored in the switches to determine the route and preserve the bandwidth and QOS requirements.
A disadvantage of all known permanent circuits is that the bandwidth remains assigned to the circuit, even when the customer has nothing to transmit. This means that the customer may have to pay higher charges than would be obtained if the bandwidth was only made available when needed. The assumption being made here is that charging is related to reserved bandwidth, and this is not necessarily correct in terms of the way public network operators may choose to charge for virtual circuits. However, it is expected that charging based on reserved bandwidth will become a significant factor in the future.
A common practice is to set up a permanent virtual circuit so that it is only available during certain hours of the day, or during certain days of the week. A difficulty with this approach is that it does not allow the customer to change the pattern of usage quickly, and it may only crudely reflect the usage required by the customer.
A second proposal has been to provide the customer with a separate communications channel to the network management plane, thereby allowing a permanent virtual circuit to be reconfigured. A difficulty with this approach is that some time delay will be incurred before the customer can start to use the virtual circuit.
A third proposal is to introduce equipment at every switching point in the network that recognises a fast resource management cell, indicating that bandwidth should now be assigned to the circuit. A difficulty with this approach is that there is no internationally agreed standard for a bandwidth-requesting cell that would be recognised by the switching equipment produced by the various manufacturers.
According to one aspect of the invention, a broadband switching system having at least one ingress for connection to a respective signal source and at least one egress for connection to a selected signal receiving system, the switching system having at least one switch for transmitting information-carrying asynchronously transferred data cells from the ingress to the egress, system control means for accepting and establishing a connection between the ingress and the egress via the said switch, and bandwidth control means arranged to detect the cells received at the ingress, and, automatically in response to such cell detection, to cause the system control means to allocate sufficient bandwidth to permit transmission of a predetermined number of cells within a predetermined time period.
Preferably, the system includes table storing means for a table associating the predetermined time period with the signal source, the time period being a predetermined maximum transmission time for a message comprising a group of the predetermined number of cells, the bandwidth control means having means for recognising the source of the detected cells thereby to identify the maximum transmission time associated with the detected cells.
In this way it is possible to alter the bandwidth available in the system for cells intended for a given path dynamically, avoiding the need to signal a request for bandwidth prior to being able to transmit cells onto the system and to provide a guaranteed maximum transmission time.
Preferably, the stored table further includes a message size value, the size value being associated with a respective signal source and being a predetermined maximum number of cells which form a message. Thus, from the stored maximum message size value and maximum transmission time, the system control means is able to calculate the cell rate (and equivalent bandwidth) required to complete the job (transmission of the message) in time. These two items of information typically are pre-registered by a customer.
The stored table may further include a bandwidth value, the bandwidth value being a predetermined maximum bandwidth allocation associated with a respective signal source.
Again, this value will typically be pre-registered and will usually correspond to the maximum transmission bandwidth capability of the signal source. This avoids the possibility of a signal source being offered a bandwidth capacity that it cannot fully use which if the system is expecting the bandwidth to be fully used, could result in the predetermined number of cells not being transmitted within the time period and also would result in incomplete use being made of the available bandwidth on the system.
Optionally, the maximum transmission time may be calculated according to a predetermined finishing time for the transmission of a message to the egress. Thus, a customer may send a message specifying a time by which it must arrive. The system control means and/or the bandwidth controller may be operable to calculate the maximum transmission time based on the current time and the predetermined finishing time.
Preferably, the system control means is arranged to calculate the bandwidth required to permit transmission of a message in the time period associated with the recognised signal source.
The bandwidth control means may be arranged periodically to monitor the number of cells of a particular message that have already been sent and to communicate the cell count to the system control means. The system control means may then decide whether the bandwidth allocated to the signal source for the transmission of that message needs to be adjusted to ensure that transmission occurs within the predetermined maximum transmission time.
It will be appreciated that a signal source may not necessarily make full use of the bandwidth allocated to it and in such a case, it is not sufficient that the system control means allocate sufficient bandwidth for the message to be transmitted in time, it is necessary for the system control means to monitor the progress which has been made in transmitting the message. The counting of cells by the bandwidth controller is one way of doing this.
The bandwidth controller may be arranged to detect periods of inactivity for a signal source, to measure the length of such inactive periods and to deem cells received at the ingress following a period of inactivity which exceeds a predetermined timeout period, to be the beginning of a new message.
Preferably, the bandwidth control means are arranged to detect the rate at which cells are supplied to the input port of the system, and bandwidth is requested from the system control means generally on the basis that the transmitting signal source should be allocated as much bandwidth as is available on the system for the indicated route. Such bandwidth will be allocated depending on the level of traffic in the system at the time the system control means is caused to allocate bandwidth. However, in the preferred embodiment of the invention, the rate at which cells are supplied to the ingress from a given signal source is monitored and, if the allocated bandwidth is more than that required to transmit the cells at the monitored rate, the system control means are caused by the bandwidth control means to allocate less bandwidth to these cells, e.g. after a predetermined time interval, thereby making bandwidth available to other customers.
The bandwidth control means may include feedback means arranged to transmit a maximum output cell rate signal back to the signal source, the indicator signal indicating a cell rate which depends on or matches the allocated bandwidth.
The bandwidth control means may be arranged also to read a bandwidth-identifying portion of the incoming cells and to cause the system control means to allocate bandwidth to the cells according to the identified bandwidth. The bandwidth control means may also be arranged to limit the rate at which incoming cells are accepted onto the system on the basis of a default bandwidth until allocation of bandwidth by the system control means has occurred. In this situation, buffering is provided in the bandwidth control means to buffer cells whilst bandwidth is being allocated. Such buffering may be used on other occasions when an signal source is transmitting at a higher rate than the rate capable of being accepted by the system at a given time. Indeed, it is preferable for the buffer to have means for detecting when it is filled to a predetermined threshold level, the feedback means being responsive to the buffer to cause a retransmission of a cell rate indicator signal to the signal source when connected to the ingress. Preferably, detection of filling of the buffer to a predetermined threshold level causes a reduced cell rate indicator signal to be transmitted to the signal source, the signal source having the facility then to reduce its cell rate to avoid buffer overflow and consequent loss of data.
The bandwidth control means of the preferred embodiment are arranged to cause automatic allocation of bandwidth for cells received from a number of signal sources, and may include an activity detector and a controller, the activity detector being operable to detect the incoming cells and to identify the signal source transmitting the detected cells, with the controller being operable in response to the said detection and identification to transmit a bandwidth request message to the system control means to allocate a level of bandwidth specifically to the cells transmitted by that identified signal source.
The activity detector may include a cell counter for counting cells received from respective signal sources coupled to the bandwidth control means. The cell count so obtained, may be used to generate charging signals for customer billing and other purposes.
The invention also includes, according to another aspect thereof, a method as claimed in claim 10 of the accompanying claims.